1. Field of the Invention
The present invention relates to a transflective liquid crystal display device and fabrication method thereof, and more particularly, to a transflective liquid crystal display device having a COT structure in which a delta film functioning as light induction material is formed on a lower substrate, and to a fabrication method thereof.
2. Description of the Related Art
Generally, a liquid crystal display device (LCD) is a flat display having advantages such as lightweight, slim profile, and low power consumption, and is widely used for portable computers such as notebook computers, office automation machines, audio/video apparatuses and the like.
The LCD controls an electric field applied to a liquid crystal material having dielectric anisotropy to transmit or shield light, thereby displaying an image. Unlike electro-luminescence (EL), cathode ray tube (CRT), light emitting diode (LED) and the like, the LCD does not generate light by itself, but uses ambient light or a backlight assembly for generating light.
Generally, the LCDs can be classified into two different categories: transmission type and reflection type depending on the usage method of light.
The transmission type LCD includes an LCD panel and a backlight, where the LCD panel has two glass substrates, and a liquid crystal layer interposed therebetween.
FIG. 1 is a cross-sectional view schematically showing a structure of the transmission type LCD according to the related art.
Referring to FIG. 1, the transmission type LCD includes: a lower substrate 102; an upper substrate 101 which faces the lower substrate 102; a liquid crystal layer 103 including liquid crystal molecules interposed between the upper and the lower substrates 101 and 102; a first polarizing plate 105 disposed on an outer surface of the lower substrate 102; a second polarizing plate 104 disposed on an outer surface of the upper substrate 101; and a backlight assembly 106 generating light to supply light.
The lower substrate 102 includes a base substrate, a plurality of gate lines and data lines arranged in a matrix configuration on the base substrate, and a plurality of thin film transistors (TFTs) disposed on the base substrate at intersection points between the plurality of gate lines and data lines and functioning as switching elements. The upper substrate 101 includes a base substrate, and a black matrix (BM) layer, a color filter layer, and a common electrode on an inner surface of the base substrate.
The first polarizing plate 105 and the second polarizing plate 104 are respectively attached on the lower substrate 102 and the upper substrate 101 such that an optical transmission axis of the first polarizing plate 105 is at a right angle to that of the second polarizing plate 104.
The arrangement of the first and second polarizing plates 105 and 104 is employed to transmit or shield light provided from the backlight assembly 106.
However, in the transmission type LCD of the related art, it is difficult to realize slimness and lightweight of the LCD due to a large volume and a heavy weight of the backlight assembly 106. Also, there is a problem that a power consumption of the backlight assembly 106 is excessively increased.
Therefore, researches for a reflection type LCD using ambient light instead of the backlight assembly 106, are actively performed. Due to its low power consumption capability, such a reflection type LCD is widely used as a portable display device such as an electronic organizer and a PDA (Personal Digital Assistant).
FIG. 2 is a cross-sectional view schematically showing a structure of the reflection type LCD according to the related art.
Referring to FIG. 2, the reflection type LCD includes: a lower substrate 202; an upper substrate 201 which faces the lower substrate 202; a liquid crystal layer 203 including liquid crystal molecules interposed between the upper and the lower substrates 201 and 202; a first polarizing plate 205 disposed on an outer surface of the lower substrate 202; a second polarizing plate 204 disposed on an outer surface of the upper substrate 201; and a reflector 206 arranged outside the first polarizing plate 205.
The lower substrate 202 includes a lower base substrate, a plurality of gate lines and data lines arranged in a matrix configuration on the lower base substrate, and a plurality of thin film transistors (TFTs) disposed on the lower base substrate at intersection points between the plurality of gate lines and data lines and functioning as switching elements. The upper substrate 201 includes an upper base substrate, and a black matrix (BM) layer, a color filter layer, and a common electrode on an inner surface of the upper base substrate.
In the reflection type LCD, an electric field applied to the liquid crystal molecules having dielectric anisotropy is controlled to transmit or block ambient light reflected by the reflector 206, thereby displaying an image.
However, in the related art reflection type LCD, when ambient light does not have a sufficient intensity (for example, the surrounding is dim), brightness level of a displayed image is lowered and the displayed information is not readable, which is problematic.
FIG. 3 is a cross-sectional view schematically showing a construction of a transflective LCD according to the related art.
Referring to FIG. 3, the transflective LCD includes: an upper substrate 311 having an upper common electrode 312 and a color filter layer formed on a first base substrate 305; a lower substrate 332, i.e., array substrate, facing the upper substrate 311 and spaced apart by a predetermined interval from the upper substrate 311; a liquid crystal layer 320 interposed between the upper substrate 311 and the lower substrate 332; and a backlight assembly 340 disposed below the lower substrate 332, for providing light toward the lower substrate 332.
On outer surfaces of the upper substrate 311 and the lower substrate 332, i.e., on an upper surface of the upper substrate 311 and on a lower surface of the lower substrate 332, upper and lower polarizing plates 313 and 336 for converting natural light into linearly polarized light by transmitting only the light parallel to their optical transmission axes are disposed.
The optical transmission axis of the upper polarizing plate 313 is at a right angle to that of the lower polarizing plate 336.
On the base substrate 305, the color filter layer (not shown) transmitting only the light having a specific wavelength, and the upper common electrode 312 that is one of two electrodes for forming an electric field are formed.
On a second base 300 of the lower substrate 332, at least one lower pixel electrode 333 that is the other electrode for forming the electric field, a passivation layer 334 having a transmission hole 331 exposing a portion of the pixel electrode, and a reflector 335 are sequentially formed.
Also, TFTs, gate lines and data lines are formed on the base substrate 300 so as to apply a predetermined voltage to the corresponding pixel electrode 333.
At this time, a region corresponding to the reflector 335 is a reflection part ‘r’ and a region corresponding to the portion of the pixel electrode, exposed by the transmission hole 331, is a transmission part ‘t’. Also, ‘d1’ is a cell gap for the transmission mode and ‘d2’ is a cell gap for the reflection mode.
In the above reflection type LCD, a phase difference δ of the liquid crystal layer 320 is obtained by the following formula:δ=Δn·d where δ: phase difference of liquid crystal, Δn: refractive index of liquid crystal, d: cell gap.
Thus, the phase difference is controllable by the refractive index of the liquid crystal and cell gap. To reduce a difference in optical efficiency between the reflection mode by the reflection part ‘r’ and the transmission mode by the transmission part ‘t’, it is required to realize a similar phase difference between the two modes.
In the transmission mode, light passes through the LCD panel once, but in the reflection mode, light passes through the LCD panel twice. Accordingly, a real effective cell gap in the reflection mode is two times greater than that in the transmission mode, so that the reflection mode has a phase difference two times greater than that of the transmission mode. To compensate for such a difference, the reflection mode and the transmission mode have different cell gaps d1 and d2 in the structure.
In the related art transflective LCD, one pixel is divided into the reflection part and the transmission part. At this point, if an area ratio of the reflection part and the transmission part becomes different in one pixel, there is a problem in that optical efficiencies in the reflection mode and the transmission mode become different even if the cell gaps of the liquid crystal layer between the two modes are made different to compensate for the phase difference between the two modes.